X-ray pictures of the Sun from SMM (Solar Maximum Mission), Yohkoh and SOHO show the existence of both large scale (e.g., coronal holes and the equatorial plasma belt) and small scale (e.g., active regions) structure in the corona. These magnetic and plasma structures can be expected to have counterparts in the solar wind, as they indeed do.
Figure 11.4: The time-averaged away/toward the Sun (positive/negative) components of the magnetic field in the
solar wind (solid lines) are compared with the disk-averaged away/toward component of the
photospheric magnetic field (dashed lines) [Severney et al., 1970].
Figure 11.4 [Severny et al., 1970; Cravens, 1997] compares the disk-averaged component of the solar magnetic field away from (positive) or toward (negative) the Sun with the corresponding time-lagged and averaged solar wind magnetic field. Three important results are apparent. First, the large scale solar field strength is well correlated with the solar wind field strength in both direction and (scaled) amplitude. This supports the corona being the source of the solar wind magnetic field. Second, the solar wind field varies at the solar rotation period (approx 27 days). Third, distinct, long-lasting intervals of uniform solar wind field direction exist, called ``sectors'' (Figure 11.5).
Figure 11.5: Illustration of sectors of magnetic field lines with different polarities
[Cravens, 1997].
Figure 11.6 shows that these magnetic properties are also associated with the solar wind speed and detailed plasma structures in the corona [e.g., Hundhausen, 1977]. Each pair of figures is for a given Carrington rotation (or rotation of the Sun). The lefthand figures show contours of the the coronal X-ray brightness overlaid with the direction of the photospheric magnetic field outward from or inward to the Sun (+ or -, respectively). Note the primarily equatorial band of intense X-ray emission, as expected from Figure 11.2, and the wavy nature of the current sheet. Positive field directions lie primarily northward of the equatorial band. The righthand figures show time variations in the speed of the solar wind measured in situ at 1 AU.
Figure 11.6: The lefthand figures show contours of the X-ray brightness of the corona and
the sense of the vertical component of the coronal magnetic field direction. The
righthand figures show the solar wind speed and the polarity of the radial
component of the solar wind magnetic field. Adapted from Hundhausen [1972].
A number of important results are apparent in Figure 11.6. First, major variations exist in the speed of the solar wind, organised into so-called ``fast'' and ``slow streams''. Second, fast solar wind streams are associated with times when the Earth is at latitudes poleward of the heliospheric current sheet. These are times when a coronal hole has moved to low heliolatitudes. Third, a given stream carries a magnetic field with the polarity (outward or inward) corresponding to the field orientation of the coronal source region. Fourth, these associations between the coronal and solar wind magnetic field and the solar wind speed are repeatable from solar rotation to solar rotation, albeit with small changes due to localized, small-scale and fast time scale coronal structures and change associated with large scale evolution of the corona itself. The overall conclusion here is that fast solar wind streams are associated with coronal holes and open field regions of the corona while slow streams come from the closed field regions primarily concentrated near the equatorial (or streamer) belt.
The Ulysses spacecraft recently finished its first passes over the Sun's north and south poles in a polar orbit. Its results allow direct testing of the above interpretations. Figure 11.7 shows the solar wind speed, magnetic polarity, and coronal brightness as a function of time and heliolatitude [McComas et al., 1998]. Clearly, the polar regions do correspond to high solar wind speed and low density while the equatorial regions correspond to slow, relatively dense solar wind speed. Moreover, regions with fast and slow streams correspond to relatively low heliolatitudes where slow streams can leave closed field regions. Finally, the magnetic field shows the expected change in polarity expected at the heliospheric current sheet.
Figure 11.7: Colour illustration of the solar wind speed and the sense of the magnetic
field's radial component observed as a function of heliolatitude by the Ulysses
spacecraft [McComas et al., 1998].
Two final remarks concerning fast and slow solar wind streams. First, the number and significance of fast and slow solar wind streams varies with the solar cycle: more long-lived fast streams are present during the declining phase of the solar cycle as coronal holes expand in size. Second, numerous other plasma properties depend on whether the stream is fast or slow, including the temperature and detailed composition of the plasma and the plasma waves present.